Functional Effects of Epilepsy Associated KCNT1 Mutations Suggest Pathogenesis via Aberrant Inhibitory Neuronal Activity.

KCNT1 (K+ channel subfamily T member 1) is a sodium-activated potassium channel highly expressed in the nervous system which regulates neuronal excitability by contributing to the resting membrane potential and hyperpolarisation following a train of action potentials. Gain of function mutations in the KCNT1 gene are the cause of neurological disorders associated with different forms of epilepsy. To gain insights into the underlying pathobiology we investigated the functional effects of 9 recently published KCNT1 mutations, 4 previously studied KCNT1 mutations, and one previously unpublished KCNT1 variant of unknown significance. We analysed the properties of KCNT1 potassium currents and attempted to find a correlation between the changes in KCNT1 characteristics due to the mutations and severity of the neurological disorder they cause. KCNT1 mutations identified in patients with epilepsy were introduced into the full length human KCNT1 cDNA using quick-change site-directed mutagenesis protocol. Electrophysiological properties of different KCNT1 constructs were investigated using a heterologous expression system (HEK293T cells) and patch clamping. All mutations studied, except T314A, increased the amplitude of KCNT1 currents, and some mutations shifted the voltage dependence of KCNT1 open probability, increasing the proportion of channels open at the resting membrane potential. The T314A mutation did not affect KCNT1 current amplitude but abolished its voltage dependence. We observed a positive correlation between the severity of the neurological disorder and the KCNT1 channel open probability at resting membrane potential. This suggests that gain of function KCNT1 mutations cause epilepsy by increasing resting potassium conductance and suppressing the activity of inhibitory neurons. A reduction in action potential firing in inhibitory neurons due to excessively high resting potassium conductance leads to disinhibition of neural circuits, hyperexcitability and seizures.

Role of CYP2E1 activity in endoplasmic reticulum ubiquitination, proteasome association, and the unfolded protein response.

In an experimental model of liver cirrhosis, marked increases in ER proteasome content in rat livers were observed 5 h after acute i.p. injection of the hepatotoxicant CCl4. To confirm the role of CYP2E1 in mediating protein misfolding/damage in the ER via its metabolism of CCl4, 293T cells stably transfected with human CYP2E1 were exposed to CCl4 and cell ER fractions assessed for ubiquitination. Increases in ER ubiquitin conjugates were noted in CYP2E1/293T cells treated with CCl4 and not in controls, suggesting these effects are CYP2E1 specific. Finally, the role of CYP2E1 in ER homeostasis was investigated by examining the unfolded protein response (UPR). When exposed to CCl4, CYP2E1/293T cells but not 293T or CYP1A2/293T cells showed rapid induction of the UPR-inducible ER chaperone BiP. Collectively, the data presented suggest that CYP2E1 is capable of inducing significant ER protein damage and stress via its catalytic activation of pro-oxidants.

Role of the von Hippel-Lindau tumour suppressor protein in the regulation of HIF-1alpha and its oxygen-regulated transactivation domains at high cell density.

Hypoxia-inducible factor-1alpha (HIF-1alpha) induction and associated transcription were investigated during high cell density, focusing on the negative regulator of HIF-1alpha expression, the von Hippel-Lindau (VHL) protein. In 293T and HeLa cells, HIF-1alpha protein levels and associated transcription were induced as cells approached confluence. To determine whether these changes were due to a deficit in nuclear VHL-mediated ubiquitination of HIF-1alpha at confluence, cells were stably transfected with VHL. Overexpression of VHL in 293T cells had no demonstrable effect on the induction and nuclear accumulation of HIF-1alpha during high cell density or associated transcription. Moreover, RCC cells stably transfected with full-length VHL failed to exhibit the cell-density-dependent induction of HIF-1alpha noted in other cell lines. Investigation of both N-terminal and C-terminal (aa 727-826) oxygen-regulated proline and asparagine hydroxylation of HIF-1alpha revealed that both are inhibited during high cell density, as determined by impaired capture of HIF-1alpha by VHL and enhanced C-terminal transactivation. Finally, cell-density-mediated induction of HIF-1alpha and GLUT1 in RCC cells could be completely reconstituted by mutations in VHL binding affinity, suggesting that cell-density dependent induction of HIF-1alpha and transactivation may underpin some of the deregulated gene expression observed in VHL disease.

Role of the C-terminal alpha-helical domain of the von Hippel-Lindau protein in its E3 ubiquitin ligase activity.

In the present study, the role of the C-terminal alpha-helical domain (amino acid (aa) 195-208) of the von Hippel-Lindau (VHL) tumour suppressor was investigated. Deletions of the VHL C-terminus up to the naturally occurring 195-Gln-Term resulted in hypoxia-inducible factor (HIF)-1alpha downregulation in renal cell carcinoma (RCC)4 cells during normoxia, suggesting that this domain is not an absolute requirement for the ubiquitination of HIF-1alpha. However, detailed investigation of the ubiquitin protein isopeptide ligase ubiquitin ligase properties of VHL revealed C-terminal deletions to cause a significant impairment of HIF-1alpha ubiquitination, which is shown to be due to a loss in high-affinity binding to the target substrate. When VHL regulation of both HIF-1alpha N- and C-terminal oxygen-dependent degradation domains (HIF-ODDD) was investigated, it was found that only ubiquitination of the C-terminal HIF-ODDD was affected by the deletion of the VHL C-terminus. When RCC4 cells expressing C-terminal truncations of VHL were exposed to graded hypoxia, differences in the induction of HIF-1alpha were observed in comparison with full-length VHL, with a shift in the maximal induction of HIF-1alpha to a higher oxygen tension. These changes were accompanied by increased glucose transporter 1 expression, p300 CH1 domain binding and HIF-mediated reporter activity. We have thus defined a role for the C-terminal alpha-helical domain of VHL in the regulation of HIF-1alpha.

Role of nuclear and cytoplasmic localization in the tumour-suppressor activity of the von Hippel-Lindau protein.

Mutations in the von Hippel-Lindau (VHL) tumour-suppressor gene result in several forms of cancer. In the present study, we investigated the role of VHL subcellular localization in its antitumour properties. We generated renal cell carcinoma (RCC) lines stably expressing either exclusively nuclear (RCC/NLS-VHL), cytoplasmic (RCC/NES-VHL) or nucleo-cytoplasmic shuttling (RCC/DeltaNES-VHL or RCC/VHL) forms of VHL and investigated several parameters linked to tumorigenesis and known to be dysregulated in VHL disease. Remarkably, although the expression of wild-type VHL is largely cytoplasmic, all of the antitumour properties of VHL tested could be reconstituted by expressing exclusively nuclear VHL.